McMillan applied "unremitting pressure" to university graduate Ms Waddell's neck for about 20 seconds while in a "worked-up and angry mood" on October 15 last year.

Jurors were asked to consider whether he killed her after discovering she was transgender or because she was unable, through her physical ailments, to perform sexually for him.

McMillan, of Bennett Road, Brighton, shook his head and tapped his fingers on the dock after the jury convicted him of murder after just over five hours of deliberation at Lewes Crown Court.

Judge Michael Lawson QC told McMillan: "What you did that night brought to an end a life which in many views was one of relentless difficulty faced with extreme courage.

"The person you killed was a person who always sought to overcome difficulties.

"On the other hand, faced with a difficulty in that flat, whatever that was, you chose to take it out on her.

"You strangled her, with relentless pressure for over 20 seconds. There was the distinction between you and her."

He added: "Once you lost control, for whatever reason and which cannot be regarded as justified, you formed the intention to destroy her, to kill her.

"It wasn't an accident and you continued to hold her round her neck, crushing the neck structures until she was dead.

"Having done that you then dragged her back into the bedroom and set fire to her and set fire to the bed on which together you had laid.

"The reason was that you were frightened that there would be some trace of your presence in that flat to prove that in fact you were the last person to have seen her."

The trial was told that McMillan - who has previous convictions for assault - went to his own home after the killing to shower and wash his clothes in a bid to cover his tracks.

Prosecutor Simon Russell Flint QC said he had drunk about seven pints of beer over the previous four hours before visiting Ms Waddell at her flat at about 10.30pm.

McMillan arranged to pay her £140 for her time after visiting the Adult Works website where Ms Waddell - who had a Master's degree in social and political thought - had four different profiles advertising her services.

He spouted expletives during the taxi ride to her one-bedroom, first-floor flat, telling the driver that all people in Cork, Ireland, were a "bunch of c****".

Mr Russell Flint said: "It was in that mood, with those parting words, that he went up to the stairs to Andrea Waddell's flat and within an hour she was dead."

When McMillan returned to his rented room after the killing, his landlord noticed a change in his demeanour.

McMillan appeared "pale-faced compared to his usual ruddy look" and he was quiet and "didn't appear to be his normal self".

He stripped off his clothes and asked his landlord if he could use the washing machine, saying he needed some clothes for work the following day.

In addition, he showered for the second time that evening and removed the laces from the boots he had worn to Ms Waddell's flat that evening.

The following day colleagues also noticed a change in his character, the court heard.

When McMillan was arrested two days after the killing, he was found to have various marks on his body, including scratches to his arm, Mr Russell-Flint said.

Following his arrest, he denied being responsible for her killing, telling officers: "I'm in my 40s and I'm Scottish and I didn't do it."

Ms Waddell's transgender background was outlined in court, detailing how she was born on June 18, 1980 as Alexander Waddell.

When she was 24, having received extensive psychological and psychiatric therapy, she underwent male to female reassignment surgery.

Ms Waddell had first shown an interest in the transgender community during her gap year.

She had travelled to Prague to teach English and became involved in the transgender scene there, Mr Russell Flint said.

When she returned to England she studied at Durham University and it was while there that she made the decision to become Andrea and change her name by deed poll.

After graduating, she successfully enrolled on to a Master's degree course at the University of Sussex, studying social and political thought.

Following her gender realignment surgery, she had to have emergency surgery for severe ulcerative colitis and in addition suffered from fibromyalgia, which causes painful muscles, abdominal problems, sleeplessness and depression.

Mr Russell Flint said that, after completing her Master's degree, she started holistic therapy.

"Being keen and eager to learn, the temptation of extra work by providing extra services seems to have seen her move into the world of a sex worker," he said.

McMillan told police in interview that there was an unnamed stranger lurking in the flat while he was there, a theory dismissed as "complete fiction" by the prosecution.

"I did not punch or strangle Andrea Waddell," he told the court in his defence. "I did not hurt Andrea Waddell in any way. I did not kill her."

He added that he did not get everything he wanted out of the half-hour session he had with her as he was unable to get an erection.

But he said this was not something that made him angry at her as it was his own problem. He also did not get annoyed with her because she refused to perform any act on him.

He said he was not a violent person or someone who often got angry or lost control.

Questioned about an earlier contact with Ms Waddell, he said he was unaware that she was the same person he met on October 15.

He had arranged for her to visit him in the hotel he was staying at in Brighton on October 2, but the night porter had refused her entry and he did not then answer her calls as he had fallen asleep.

He said he was woken up a while later by his mobile phone beeping to signal he had a text message and he saw that he also had "three or four missed calls" from her.

McMillan said he decided to call her to apologise for not answering his phone, and they spoke for around six minutes.

He said she seemed more upset at the way she was treated at the hotel than at him but he offered to reimburse her for her time and taxi fare to make up for it anyway. "We parted on good company," he added.

In a statement, Ms Waddell's family expressed relief that McMillan would no longer be free to destroy another life.

They said: "We could never be truly happy with any outcome from this trial as nothing can bring back our beautiful Andrea but we are relieved and satisfied that Neil McMillan has been found guilty of her horrific murder.

"This verdict cannot take away the terrible sense of loss and of pain that will be with us forever but it is right and proper that justice is done - both for Andrea's sake and to ensure this truly evil man cannot destroy any other lives in the future."

They added: "As a family, we can now remember Andrea, her passions and the causes she fought for during her life, knowing that the person who callously extinguished that life is behind bars.

"We hope that everyone who has read about Andrea over the past seven months will be inspired by her, as we all are, to live their lives true to themselves, not to judge other people for their differences and to strive to make the world a better and more just place."

Detective Chief Inspector Adam Hibbert, from Sussex Police's major crime branch, said: "Andrea was a young woman who had her whole life ahead of her and it was cruelly cut short.

"Our thoughts are with her family and friends at what is still a very difficult time.

"She was brutally killed in her own home by Neil McMillan, who then attempted to dispose of evidence by setting fire to her property and washing his clothing.

"He tried to cover his tracks to avoid being caught, but compelling CCTV footage together with evidence from witnesses means he is now serving a prison sentence for his terrible crime.

"His actions on the night of the killing and his subsequent interviews with police have been exposed as devious and deceitful.

"Due to the nature of Andrea's work, people were understandably reluctant to come forward to speak to police but they bravely did so.

"We are extremely grateful for the support we received from the local LGBT (lesbian, gay, bisexual and transgender) and sex worker communities - without them, Andrea and her family would not have received the justice they deserve."